The present invention relates generally to a device for generating and measuring the flow of gas and, more particularly, to a device which is adapted for use in determining air infiltration in buildings and the like. While the device will be discussed hereinafter primarily with reference to the latter use, it should be understood that its utility is not thereby so limited.
The recent escalations in the costs of petroleum products and other types of fuel have focused great national attention on means for conserving energy. One area of significant potential savings in energy is in the energy used to heat and cool buildings such as residential homes. The two major sources of energy loss in buildings are thermal conduction through walls, ceilings and floors, and infiltration of air where, for example, warm inside air is displaced by cold outside air through cracks, holes and other openings in the structure of the building. Techniques for minimizing energy losses by conduction in both new and old buildings are well known (insulating materials of various kinds, storm windows and doors, etc.) and has been well publicized both by public service agencies and commercial manufacturers. Procedures to measure conductive heat loss from buildings such as infra-red scanning are also available.
On the other hand, it is not nearly as generally appreciated that in most buildings, new or old and with or without adequate insulation, air infiltration is still a major source of heat loss. According to one estimate, from 15 to 67% of the total heating energy utilized in residential buildings is due to the infiltration of air. Furthermore, with the exception of weatherstripping around doors and windows and exterior caulking, it is still not general practice in new residential construction to build with a goal of reducing air infiltration to the minimum practical level. Older houses, built in the era of cheap energy, are even worse in this regard.
Techniques to minimize air infiltration are known and in general are relatively inexpensive to accomplish. In order to promote the more widespread application of these techniques and to thereby capitalize on the potential energy savings realized by the use of these techniques, a simple and economical testing device to measure air infiltration in residential homes and other buildings should be made widely available. To be effective, such a testing device should be able to quickly determine how serious a problem air infiltration is in a given structure, and also should be capable of indicating the effectiveness of the steps taken to reduce the air infiltration. In addition, it would also be very useful to be able to identify sources of air leakage in the structure from use of the testing device.
Two instrumental techniques have been described in the literature for estimating the resistance of buildings to air infiltration: (a) a gas diffusion method and (b) a pressurization method. In the latter technique, a device having an exhaust type fan is utilized to establish a given pressure differential between the interior and exterior of the building. From a determination of the flow rate of the air being exhausted from the building, the amount of air exhausted per unit of time can be calculated, e.g., cubic feet per minute (cfm). Then, using the volumetric capacity of the building (total floor area).times.(ceiling height), the number of air turnovers per hour (ATPH) is then calculated by the following formula; ##EQU1##
ATPH is conventionally used as an index of the resistance of the house to air infiltration. For example, Sweden has incorporated this index of air infiltration into its national building code for all new construction, in some cases the standards being as low as 1.0 ATPH at 0.2 inches wc pressure. One proposed standard for the United States is that for an adequately tight house, ATPH should be between about 1.5 and 5.0 at 0.1 inches wc pressure. It is generally recognized that values below about 1.5 indicate the building may be too tight and therefore subject to problems from the buildup of excessive humidity, odors and/or hazardous fumes. Above a value of about 5.0, air infiltraton is apt to be an increasingly high factor in heating or cooling costs for the building. For example, in a relatively new residential home with a heat-pump type heating system, a reduction in the ATPH down to a value about 3 from an unknown initial level has been accompanied by an almost 50% reduction in electrical consumption for heating compared on a degree-day basis with earlier experience.
Another procedure which has been proposed for expressing the results of the pressurization method for testing air infiltration is by determining a parameter known as the Equivalent Leakage Area (ELA) for the building. ELA is calculated by a graphical procedure using data on air leakage rate versus the differential pressure outside and inside the building, .DELTA.p(out/in). This procedure has been used principally in tests with the "Blower Door" testing device described below, but it could also be used with the present invention, if desired.
One known testing device used in the above-mentioned pressurization method for determining air infiltration comprises a tubular section about five feet long and eighteen inches in diameter. Fitted at one end of the tubular section is a tube-axial fan which is belt-driven by a variable-speed, 3/4 horsepower electric motor mounted adjacent to the tube. At the other end of the tubular section is a short transitional section to a square cross-sectional shape which is used for mounting the device in an opening such as a door or window in the building to be tested. The interior of the tube is provided with a honey-comb type baffle arrangement to provide laminar air flow therethrough and an array of Pitot tubes are located at a number of points across the tube cross-section for measuring air velocity. During use of the device, the square end of the tubular section is mounted from the exterior of the building into a convenient window or door opening, the space surrounding the device is sealed, and the other end of the device containing the motor and fan is maintained in place by an adjustable vertical support. Accessory equipment for the device includes an inclined-tube manometer to measure pressures at the Pitot tube locations, and another pressure gauge to measure the pressure difference between the inside and outside of the building.
Two uses of the above-mentioned testing devices are described: first, to measure the resistance of the building to air infiltration, and second, to provide assistance in determining the locations of air leakage. For the first use, the speed of the fan drawing air from inside the building is adjusted to produce an outside-to-inside pressure difference of 0.1 inches of water column (wc). The average air velocity inside the tube is then determined from the Pitot tube pressure readings, which in turn permits calculation of the rate (CFM) at which air is being exhausted from the building in order to maintain the 0.1 inch wc pressure differential. With the volumetric capacity of the particular building being tested, an APTH value can then be determined from the previously-mentioned formula.
By increasing the fan speed of the device to generate a maximum of 0.4 inches wc outside-to-inside pressure differential, sources of air leakage in the building can be more easily located. Thus, for example, one can readily locate leakage by simply feeling for drafts in various locations within the building.
While the above-described device is generally adequate for the purpose for which it was designed since it is well-suited for precise, absolute measurements in a scientific study of various factors affecting air infiltration, it may not be considered attractive for widespread use by builders and home-owners for a variety of reasons. For example, the device is relatively expensive, with an estimated cost for materials alone above a thousand dollars. In addition, the size and weight distribution of the device make it relatively inconvenient to transport and it is difficult for one person to mount the device in position for conducting a test. Furthermore, the necessity of converting the multiple Pitot tube pressure readings to air flow in cfm involves a fairly complicated and time-consuming calculation.
Another test device which has been used to measure air infiltration by the pressurization method is a device known as the "Blower Door" which was developed at Princeton University. Air flow rate is determined from measurements of fan rpm, .DELTA.p(out/in), and air density, by applying a relationship previously established by calibration. This device may have several disadvantages in regard to its large-scale manufacture and routine use. Each device would probably have to be calibrated individually because calibration is sensitive to small variations in dimensions of the fan and other mechanical elements. Accuracy of flow measurement tends to be reduced by the fact that .DELTA.p(out/in) is a factor in the calibration relationship. The device is somewhat inconvenient for use by one person, since the fan assembly and motor alone weigh about eighty pounds.